Aromatic hydrocarbons, which refer to the hydrocarbons containing benzene ring structure, are one of basic products and basic raw materials, mainly including benzene, toluene, xylenes, ethylbenzene and the like, and are widely used in the production of chemical fibers, plastics, rubber and other chemical products. In recent years, with the worldwide continuous expansion of polyester production, the demand for PTA (terephthalic acid) and PX (p-xylene) has increased rapidly.
Traditionally, aromatics can be produced from petroleum. There are two technological routes for production of aromatics from petroleum. The first one involves naphtha catalytic reforming and aromatics are subsequently obtained by extraction from complex mixtures. The second one: the pyrolysis gasoline is hydrogenated and aromatic hydrocarbons with low value are extracted, which are then converted into high value added aromatic productions. The technology for producing aromatics from the petroleum routes are mature, but restricted by thermodynamic equilibrium, the PX content in the aromatic productions is low. Thus, further processing and enriching are needed through adsorption separation or crystallization separation, in order to reduce the loss of raw materials and energy consumption, and hence reducing the production cost of PX through the petroleum routes. Another source of the raw materials for production of the aromatic hydrocarbons is coal. The aromatics can be produced through coal chemical technologies (including coal gasification, coal coking, coal liquefaction, etc.) combined with petrochemical technologies, such as a technology for producing PX through toluene and methanol methylation. With the development of the oil refining industry, the proportion of aromatic hydrocarbons using petroleum as raw material is gradually increased, accounting for more than 98% outside of China and accounting for more than 85% in China. In recent years, the production capacity of aromatic hydrocarbons in China has increased significantly and reached a certain scale, but it still cannot meet the actual market demand. The aromatics and the ethylene are the two most important products in petro and chemical industries in China, which can be used to fabricate a range of other important chemicals and polymers, such as polyethylene, polypropene, styrene, polyester and nylon. Before 2004, due to the low production capacity of benzene downstream products in China, the products can basically meet the demand and some products are exported. However, the production capacity of the benzene downstream products in China has increased rapidly in the past two years, and especially with the large-scale construction of the production apparatuses of styrene, phenol, aniline, cyclohexanone and the like, the demand in the benzene market is increased greatly. Therefore, China turns into a pure importing country from an exporting country of benzene. It is expected that in the next few years, Chinese benzene will not meet the downstream market demand. For China, it is of great significance to develop a new technology for synthesis of aromatic hydrocarbons from resources other than petroleum, particularly under the current energy supply situation of “rich coal reserve, but deficiency in oil and natural gas”. The technology of synthesis of aromatics through methanol and dimethyl ether or through synthesis gas directly opens up a new technology route for BTX production from coal. It can effectively relieve the contradiction between scant supply of the aromatic hydrocarbons in China and excess capacity of methanol, and has a good prospect.
The technology of synthesizing aromatic hydrocarbons by catalytic reaction of methanol or dimethyl ether has been explored, and it has been demonstrated up to pilot plant tests. However, this technology needs multiple reaction steps. Firstly, the synthesis gas is converted into methanol or dimethyl ether, etc. Then methanol or dimethyl ether is converted to aromatics via the so-called MTA process. In this process, the H2/CO ratio in the synthesis gas shall be 2 for methanol synthesis. If the synthesis gas comes from coal gasification and the H2/CO proportion is generally 0.5-1, then the H2/CO proportion must be adjusted to 2 by the water-gas shift process, which is a high energy consumption and high water consumption process. In addition, the technology is susceptible to fluctuations in the methanol market. It was reported that the aromatic hydrocarbons can be possibly synthesized via direct conversion of the synthesis gas using the Fe/MnO-ZnZSM-5, which converted the synthesis gas into the light olefin intermediates via Fischer-Tropsch reaction and then to aromatics on the zeolite. It showed a higher zinc content benefited the formation of the aromatic hydrocarbons products and reducing the formation of the gas phase low carbon hydrocarbons. However, the selectivity of the aromatics in that process was low, which was only about 53% (Chinese Journal of Catalysis, 2002, 23, Vol. 4, 333-335). Fe—Pd/HZSM-5 was also studied for synthesis of the aromatics. Under the temperature of 340° C., pressure of 8.62 MPa and space velocity of 3000 h−1, CO conversion reached 75.7%, but the selectivity of the aromatics in liquid phase products was only 32.0% (Energy Fuels, 2014, 28, 2027-2034). Pd/SiO2+HZSM-5 or H-mordenite catalyst was also used to synthesize the aromatics through the route from methanol. The influence of different reaction conditions on the product distributions and yield were discussed. The aromatics selectivity was also low, only about 50%, and the products are mainly tetramethylbenzene and pentamethylene (Journal of Catalysis, 1984, 87, 136-143). Therefore, it is particularly urgent to develop a technology for synthesis of aromatics from synthesis gas with a high selectivity in order to alleviate the scant supply of the aromatics in China.